Retrieves the MIME type of the resource pointed by uri.
In the event the URI cannot be found, NULL is returned and
error is set to G_BOOKMARK_FILE_ERROR_URI_NOT_FOUND In the
event that the MIME type cannot be found, NULL is returned and
error is set to G_BOOKMARK_FILE_ERROR_INVALID_VALUE
a valid URI
a newly allocated string or NULL if the specified
URI cannot be found.
GException on failure.